1. Field of the Invention
The embodiments of the invention relate to an antenna array having a WAIM layer for impedance matching for large skew angles. In particular, the antenna array includes an antenna baseplate having a plurality of antenna elements arranged in a regular grid and a dielectric wide angle impedance match (WAIM) layer arranged in front of the antenna elements for impedance matching for large skew angles.
2. Discussion of Background Information
One phenomenon that is often observed in the transmission behavior of an antenna array while the main beam is being electronically scanned is the difference in the transmission level depending on the direction to which the antenna is skewed. Normally, an antenna has a defined polarization alignment, for example vertical or horizontal polarization. In order to explain this phenomenon, it is sufficient to electronically skew the main beam of an antenna array in an imaginary form along two planes (vertical & horizontal). If the vector of the emitted electrical field strength is within the skew plane, defined as being formed from the skew direction and normal to the antenna, the term transverse magnetic polarization (TM) is used. If the vector of the electrical field strength is at right angles to this plane, the term transverse electrical (TE) is used. All other possible polarization states can be broken down into these two polarization components. In principle, conventional antenna arrays (as well as other structures of a related type such as dielectric or frequency-selective radomes) have a tendency to form a poorer transmission level in TE than in TM as the skew angle increases.
A so-called wide angle impedance match (WAIM) layer, which is arranged in front of the antenna elements, can counteract this effect. With respect to the two polarization cases of TE and TM, the WAIM layer acts analogously to an equivalent line model of the antenna as a parallel-connected capacitance, whose relative susceptance (with respect to the characteristic impedance) varies with the skew angle θ. For the case of TE polarization, this change takes place with the factor 1/cos(θ), but with the factor cos(θ) for the case of TM polarization, provided that the dielectric constant of the WAIM layer is sufficiently high, and the thickness of the WAIM layer is sufficiently thin. If the WAIM layer is suitably designed, the described reciprocity of the factors now leads to the transmission levels of the antenna being matched to one another, between TE and TM polarization, during skewing. This applies to all possible skew angles within a technical sensible range from, for example, θ=0° to θ=60°. This matching then results in the normally desired broad individual polar diagrams of antenna elements of an antenna array in all the important section levels.
The solutions used until now have been based essentially on the theoretical works by Magill & Wheeler (E. Magill and H. Wheeler, “Wide-angle impedance matching of a planar array antenna by a dielectric sheet,” IEEE Transactions on Antennas and Propagation, Vol. 14, No. 1, pages 49-53, 1966), the disclosure of which is expressly incorporated by reference herein in its entirety. A WAIM layer carries out the purpose of transmission matching between TE and TM polarization only if it is kept at a short but well-defined distance from the antenna elements in the antenna array.
The standard solution for production of the necessary physical separation is the use of RF foam materials, as described, e.g., in U.S. Pat. No. 7,580,003 B1, the disclosure of which is expressly incorporated by reference herein in its entirety. While the availability of foams such as these does not represent a problem, a range of disadvantages occur in the course of use of such foams:                Hygroscopy: many foams have a tendency to absorb moisture from the environment over time, and this leads to a major change in the dielectric characteristics. The consequence of this is complex measures for encapsulation of the foam layer.        Tolerances: foam layers with a thickness of a few millimeters can be produced only in a moderate tolerance band.        Adhesive bonding: in principle, suitable standard materials for the WAIM layer (commercially available RF printed circuit board materials with a high dielectric constant, e.g., Rogers RT/duroid 6010) contain polytetrafluoroethylene (Teflon), which represents a problem in terms of long-lasting and reliable adhesive bonding to the foam material. It is admittedly in principle technically feasible to produce such adhesive bonds, but only with complex measures such as plasma activation of the WAIM components which contain Teflon.        
U.S. Pat. No. 3,605,098, the disclosure of which is expressly incorporated by reference herein in its entirety, describes an antenna array in which there is a separate WAIM element in front of each antenna element. A WAIM element such as this in each case includes a WAIM layer parallel to the plane of the antenna elements, as well as spacers on which the WAIM layer is arranged.
MCGRATH D T: “Accelerated periodic hybrid finite element method analysis for integrated array element and radome design, PHASED ARRAY SYSTEMS AND TECHNOLOGY, 2000. PROCEEDINGS. 2000 IEEE INTERNATIONAL CONFERENCE ON DANA POINT, CA, USA 21-25 MAY 2000, PISCATAWAY, NJ, USA, IEEE, US, 21 May 2000 (21 May 2000), pages 319-322, XP010504600, DOI: DO1: 10.1109/PAST.2000.858965, ISBN: 978-0-7803-6345-8 describes an antenna array having waveguide antenna elements, with the waveguide antenna elements having dielectric filling elements in order to specifically vary the radiation characteristics of the antenna. The dielectric filling elements project out of the antenna. A WAIM layer is arranged on these projecting dielectric filling elements. The disclosure of this document is expressly incorporated by reference herein in its entirety